1. Field of the Invention
This invention relates to a rubber composition and a pneumatic tire using such a rubber composition as a tread rubber, and more particularly to a rubber composition capable of using as a tread rubber to highly balance the steering stability, wear resistance and fracture properties of a tire.
2. Related Art
With the advance of high power performance of recent vehicles comes the demand better steering stability, particularly a tire having excellent steering stability on a dry road surface. On the other hand, it is important to sufficiently ensure the wear resistance and fracture properties of the tire from a viewpoint of economical efficiency and safety. In this regard, various techniques have been developed for improving the steering stability of the tire. In general, it is known that a loss characteristic above room temperature (tan δ) is an important indicator for developing the rubber composition for the steering stability of the tire. In order to improve the steering stability of the tire, it is effective to increase a hysteresis loss above room temperature of the rubber composition used as the tread rubber of the tire.
As the technique of increasing the hysteresis loss of the rubber composition, a method of using a liquid polymer having a weight average molecular weight of several tens of thousands is known (see JP-A-61-203145 and JP-A-63-101440). However, the liquid polymer has a problem that, although the weight average molecular weight is as low as tens of thousands, the amount of double bonds having a crosslinking property is large, and thus, a part of the liquid polymer is incorporated into a matrix rubber by crosslinking with the matrix. Hence, sufficient hysteresis loss is not developed.
On the contrary, a method to increase hysteresis loss of the rubber composition is proposed wherein the double bond in the liquid polymer is saturated by hydrogenation so as not to be incorporated into the matrix by crosslinking of the liquid polymer. In this case, there is a problem that as the hydrogenating ratio of the double bonds in the liquid polymer becomes too high, the compatibility of the hydrogenated polymer with the matrix rubber can not be ensured and hence the fracture properties of the rubber composition lower.